selftests: mptcp: add infinite map mibs check

This patch adds a function chk_infi_nr() to check the mibs for the
infinite mapping. Invoke it in chk_join_nr() when validate_checksum
is set.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>